<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .left{
      float: left;
      width: 200px;
      background-color: aqua;
      height: 300px;
    }
    .right {
      overflow: hidden;
      height: 300px;
      background-color: blueviolet;
    }
  </style>
</head>
<body>
  <div class="left">左边侧栏</div>
  <div class="right">右自适应边侧栏</div>
</body>
</html>